A material's BRDF (bidirectional reflectance distribution function) represents its reflectance, i.e., how it shadows and scatters light incident at a single point on its surface. In general, the BRDF is a four-dimensional (4D) function, depending on the direction of the light and the view relative to the surface normal vector. Many materials are isotropic, as their reflectance does not change if a flat uniform sample is rotated azimuthally around its normal vector. Isotropic materials can be parameterized by a three-dimensional (3D) rather than a 4D set of measurements. However, these tend to be substantially cumbersome representations, more particularly if reflectance varies spatially.